The present invention relates to a catalyst for the purification of the exhaust gases of internal combustion engines by means of oxidation and/or reduction. In a further aspect, the present invention relates to a method of preparation of the catalyst and usage thereof. The catalysts can contain the platinum group metals platinum, palladium and rhodium individually or in combination. According to one aspect, the catalyst can be used, depending on composition, as the reduction catalyst of a double-bed reactor. In this case the exhaust gas is first supplied to a reducing catalyst and then after the admixture of air to an oxidizing catalyst. In another aspect, the catalyst can be used as a multifunctional catalyst for the simultaneous conversion of the oxidizable and the reducible components of the exhaust gas.
Motor exhaust gas catalysts, especially those with high cerium oxide content, are able to bind sulfur oxides under oxidizing exhaust gas conditions. However, during periods of travel when conditions change to reducing exhaust gases, the amounts of sulfur oxide stored in this manner can be released again in part as hydrogen sulfide.
Since motor exhaust gases can contain sulfur dioxide due to a use of sulfur-containing fuels and since the exhaust gas catalyst encounters both oxidizing and also reducing conditions depending on the manner of driving, hydrogen sulfide can be emitted when using auto exhaust gas catalysts. As a result, the H.sub.2 S odor limits can be distinctly exceeded for a brief period.
It has been known for some time that nickel containing catalysts exhibit only very slight emissions of hydrogen sulfide. However, the use of nickel as a component in motor exhaust gas catalysts is controversial on account of its carcinogenic action. It should therefore fundamentally be avoided.
There is therefore a need, for environmental reasons, for nickel-free motor exhaust gas catalysts with a reduced tendency to emit hydrogen sulfide.
The present invention is an improvement with respect to the general teaching of DE-PS No. 29 07 106 for the formulation of exhaust gas purification catalysts. This prior art publication in essence describes an exhaust gas catalyst with an active phase consisting of 0.03 to 3% by weight platinum and rhodium and optionally nickel which is applied onto aluminum oxide of the transition series and is obtained by impregnating the optionally lattice-stabilized carrier with an aqueous solution of a salt of platinum and rhodium and optionally of nickel. The resulting product is dried and treated in a gas current containing hydrogen at 250.degree. to 650.degree. C. The catalyst also contains 5 to 15% by weight of a mixture of CeO.sub.2 and ZrO.sub.2 in weight ratios of 20:80 to 80:20 and 1 to 10% by weight Fe.sub.2 O.sub.3. This mixture is impregnated before the addition of the noble metals with an aqueous solution of cerium salt, zirconium salt and iron salt or mixed with their oxides and subsequently tempered 30 to 180 min. in air at 500.degree.-700.degree. C. Without a nickel content, these catalysts result in an exhaust gas with a distinct H.sub.2 S odor, so that the problem arose of eliminating the unwanted exhaust gas odor with other means. It is possible, by means of a modification of the nickel-free composition, which can extend in the same manner to the oxide component and the finished catalyst, and especially by also using zinc oxide in the known formulations, which have optionally been broadened in their amount, to eliminate practically completely this deficiency. Even in the case of elevated cerium oxide content of the catalyst, which is especially critical as regards the tendency to H.sub.2 S emission this problem can be largely avoided.